Light-emitting element, method of producing light-emitting element, and display device

ABSTRACT

A light-emitting element where a positive electrode is formed on the surface of a transparent substrate; a hole transport layer is formed on the surface of the positive electrode; and a light-emitting layer made of quantum dots is formed on the surface of the hole transport layer. The light-emitting layer has a light-emitting region that emits light of a first predetermined wavelength in which a surfactant is present on the surface of the quantum dots and a non-light-emitting region that does not emit light in which a surfactant is absent on the surface of the quantum dots. A second light-emitting layer that emits light of a second predetermined wavelength is formed on the surface of the light-emitting layer, and a negative electrode is formed on the surface of the second light-emitting layer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of International applicationNo. PCT/JP2011/060956, filed May 12, 2011, which claims priority toJapanese Patent Application No. 2010-118212, filed May 24, 2010, theentire contents of each of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a light-emitting element, a method ofproducing a light-emitting element, and a display device, and moreparticularly to a light-emitting element having at least onelight-emitting layer formed of quantum dots, a method of producing thelight-emitting element, and a display device such as a full-colordisplay using the light-emitting element.

BACKGROUND OF THE INVENTION

Quantum dots which are ultrafine particles having a particle size of 10nm or less are excellent in confinement of carriers (electrons, holes),so that excitons can be easily generated by recombination ofelectrons-holes. For this reason, light emission from free excitons canbe expected, and light emission having a high light-emission efficiencyand a sharp light-emission spectrum can be realized. Also, because thequantum dots can be controlled in a wide wavelength range using thequantum size effect, a light-emitting element containing quantum dots ina light-emitting layer has been attracting attention and has beeneagerly studied and developed in recent years.

For example, Patent Document 1 proposes a method of producing anelectroluminescence (hereafter referred to as “EL”) element in which alight-emitting layer containing quantum dots is patterned by using thephotolithography method, as shown in FIG. 8.

In this Patent Document 1, a first electrode layer 102 electricallyinsulated via an insulating layer 101 is formed on a substrate 103; ahole injection layer 104 is formed thereon; and a photoresist layer 105is formed in a predetermined pattern so that the photoresist layer of alight-emitting region may be removed, as shown in FIG. 8( a).

Next, a light-emitting layer 106 is formed by using an applicationliquid containing quantum dots having a silane coupling agentcoordinated thereto and a hole-transporting material, as shown in FIG.8( b).

Next, the photoresist layer 105 is exposed via a photomask, developedthereafter with a photoresist developing liquid, and washed, whereby theremaining photoresist layer 105 is removed, and the light-emitting layer106 on the photoresist layer 105 is lifted off, and thus alight-emitting layer 106 a having a pattern form is formed, as shown inFIG. 8( c).

Finally, then, a second electrode layer 107 is formed on thelight-emitting layer 106 a, as shown in FIG. 8( d).

Also, Patent Document 2 proposes a method of producing an EL element inwhich a light-emitting layer containing quantum dots is patterned byusing a layer in which the wettability changes by action of aphotocatalyst accompanying energy radiation, as shown in FIG. 9.

In this Patent Document 2, a first electrode layer 112 electricallyinsulated via an insulating layer 111 is formed on a substrate 113, anda wettability changing layer 114 is formed thereon, as shown in FIG. 9(a). Here, the wettability changing layer 114 contains a photocatalystand is constituted in such a manner that the wettability changes byenergy radiation such as ultraviolet rays.

Next, ultraviolet rays are radiated onto the wettability changing layer114 via a photomask. Then, by action of the photocatalyst contained inthe wettability changing layer 114, the wettability changes in theirradiated part of the wettability changing layer 114 so that thecontact angle to the liquid may decrease thereby to form a lyophilicregion 115, and a liquid-repellent region 116 in which the wettabilitydoes not change is formed in the non-irradiated part, as shown in FIG.9( b).

Then, when an application liquid containing quantum dots having a ligandsuch as a silane coupling agent coordinated thereto is applied onto thewettability layer 114, the application liquid is repelled in theliquid-repellent region 116 while the application liquid adheres in thelyophilic region 115, as shown in FIG. 9( c), whereby a light-emittinglayer 117 is formed on the surface of the lyophilic region 115.

Finally, then, a second electrode layer 118 is formed on thelight-emitting layer 117, as shown in FIG. 9( d).

-   Patent Document 1: Japanese Patent Application Laid-open (JP-A) No.    2009-87760 (claim 1, paragraph [0026], and FIG. 1)-   Patent Document 2: Japanese Patent Application Laid-open (JP-A) No.    2009-87781 (claim 1, paragraphs [0034] to [0038], and FIG. 1)

SUMMARY OF THE INVENTION

However, in Patent Document 1, the light-emitting layer is patterned bythe photolithography method. Therefore, in dissolving and removing theunnecessary photoresist 105 on the hole injection layer 104, thephotoresist 105 cannot be completely removed, thereby raising a fearthat a residue may be present on the hole injection layer 104. Then,when the residue of the photoresist 105 is present on the hole injectionlayer 104, there is a fear of inviting a decrease in the light-emissioncharacteristics.

Also, in Patent Document 1, the patterning is carried out with use of aphotoresist. Therefore, for the hole injection layer 104, there is aneed to select a material having a chemical resistance against exposureto a series of photolithography processes, thereby raising a problem inthat there is a restriction on the material selection.

Also, in Patent Document 2, the wettability changing layer 114 having norelation to the light-emission characteristics is interposed between thelight-emitting layer 117 and the first electrode layer 112, so that theresistance of the EL element will be high, thereby raising a problem inthat the driving voltage will be high.

Moreover, in Patent Document 2, the contact angle to the liquid changesgreatly at the boundary between the irradiated part and thenon-irradiated part of the ultraviolet rays, so that the change in filmthickness at such a part will be large. For this reason, there is a fearthat unevenness may be generated in the light-emission characteristics.

The present invention has been made in view of such circumstances, andan object thereof is to provide a light-emitting element in whichpatterning of a light-emitting region can be carried out easily withoutthe need for cumbersome steps, and good light-emission characteristicscan be obtained at a high efficiency and with a low cost, as well as amethod of producing a light-emitting element and a display device usingthis light-emitting element.

With regard to the quantum dots which are ultrafine particles of ananometer level, a surfactant is coordinated therearound in order toavoid agglomeration of the quantum dots with each other. This surfactantcan inactivate the surface defects of the quantum dots and can confinethe carriers (holes, electrons) effectively within the quantum dots byvoltage application, whereby the holes and electrons are recombinedwithin the quantum dots to allow exciton light-emission at a highefficiency.

On the other hand, when the surfactant on the quantum dot surface isremoved, the surface defects are not inactivated, so that the excitonsare trapped by the surface defects, and the excited energy isdeactivated by thermal radiation to bring about a non-light-emissionstate. Then, this surfactant can be easily removed by performing athermal treatment at a temperature around the boiling point.

Therefore, a light-emitting region and a non-light-emitting region canbe formed by performing a thermal treatment on a specific region of thequantum dot layer, whereby a light-emitting element having goodlight-emission characteristics can be obtained easily and efficientlywithout using the photolithography method as in Patent Document 1 orwithout providing a wettability changing layer as in Patent Document 2.

The present invention has been made based on such a finding, and alight-emitting element according to the present invention is alight-emitting element comprising at least one or more light-emittinglayers containing quantum dots interposed between electrodes, wherein atleast one or more layers of the light-emitting layers have alight-emitting region in which a surfactant is present on the surface ofthe quantum dots and a non-light-emitting region in which a surfactantis absent on the surface of the quantum dots.

Also, in the light-emitting element of the present invention, it ispreferable that the non-light-emitting region is made by a thermaltreatment.

Also, when an electron transport layer having a light-emitting functionis formed as a second light-emitting layer on the surface of thelight-emitting layer, the leakage current passing through the surfacedefects from the quantum dots in the non-light-emitting regionincreases, whereby electric charge is concentrated in the secondlight-emitting layer to allow the second light-emitting layer to emitlight. Therefore, light of a respectively different wavelength can beoutput from both the light-emitting layer and the second light-emittinglayer.

Specifically, in the light-emitting element of the present invention, itis preferable that a second light-emitting layer is formed on thesurface of the light-emitting layer.

Also, in the light-emitting element of the present invention, it ispreferable that the second light-emitting layer is an electron transportlayer.

Further, in the light-emitting element of the present invention, it ispreferable that the light-emitting region and the second light-emittinglayer each emit light having a respectively different wavelength.

Also, when the light-emitting layer is formed of a laminated body ofquantum dot layers having different particle sizes and thelight-emitting layers are laminated so that light emission can be madefrom the light-emitting region of each of the light-emitting layers,light having a respectively different wavelength can be output from eachof the light-emitting layers by the quantum size effect.

Specifically, in the light-emitting element of the present invention, itis preferable that the light-emitting layer is formed of a laminatedbody of two or more kinds of quantum dot layers having different averageparticle sizes, and the light-emitting layers are laminated so that atleast a part of the non-light-emitting region and at least a part of thelight-emitting region may overlap with each other.

Further, in the light-emitting element of the present invention, it ispreferable that each light-emitting region of the light-emitting layeremits light having a respectively different wavelength.

Also, in the light-emitting element of the present invention, it ispreferable that an electron transport layer is formed on the surface ofthe light-emitting layer.

Further, in the light-emitting element of the present invention, it ispreferable that a hole transport layer is interposed between oneelectrode and the light-emitting layer.

Also, in the light-emitting element of the present invention, it ispreferable that the quantum dots have a core-shell structure.

Also, a method of producing a light-emitting element according to thepresent invention is a method of producing a light-emitting elementincluding at least one or more light-emitting layers containing quantumdots interposed between electrodes, the method comprising a quantum dotfabricating step of fabricating at least one or more kinds of quantumdots having a surfactant coordinated on the surface; a quantum dot layerforming step of forming at least one or more quantum dot layers by usingeach of the quantum dots fabricated in the quantum dot fabricating step;and a surfactant removing step of performing a thermal treatment on aspecific region of at least one quantum dot layer among the quantum dotlayers to remove the surfactant on the specific region, and thus alight-emitting layer having a light-emitting region and anon-light-emitting region is fabricated.

Further, in the method of producing a light-emitting element of thepresent invention, it is preferable that a second light-emitting layeris formed on the surface of the light-emitting layer.

Also, in the method of producing a light-emitting element of the presentinvention, it is preferable that the second light-emitting layer is anelectron transport layer.

Also, in the method of producing a light-emitting element of the presentinvention, it is preferable that two or more kinds of quantum dotshaving different average particle sizes are fabricated in the quantumdot fabricating step, and the quantum dot layer forming step and thesurfactant removing step are carried out for plural times so that atleast a part of the non-light-emitting region and at least a part of thelight-emitting region may overlap with each other by using these quantumdots to form two or more light-emitting layers having differentlight-emission wavelengths.

Also, in the method of producing a light-emitting element of the presentinvention, it is preferable that an electron transport layer is formedon the surface of the uppermost light-emitting layer among thelight-emitting layers.

Also, in a display device according to the present invention, numerouslight-emitting elements described above are arranged in an array form.

According to the light-emitting element and method of producing alight-emitting element of the present invention, a light-emitting regiondefining light emission and non-light-emission can be patterned simplydepending on whether a surfactant is present or not on the surface ofthe quantum dots contained in the light-emitting layer.

Specifically, the light-emitting region can be patterned without using acumbersome method such as the photolithography method. Therefore, alight-emitting element having good light-emission characteristics can beobtained at a high efficiency and with a low cost without the presenceof a residue of the photoresist in the light-emitting layer and withoutthe need for consideration of chemical resistance at the time ofmaterial selection. Also, the film thickness of the light-emitting layercan be controlled easily in the film forming process, so that the filmthickness can be formed uniformly or approximately uniformly, andunevenness is not generated in the light-emission characteristics.Moreover, the element will not have high resistance as in PatentDocument 2, so that the element can be driven by application of lowvoltage.

Also, when a second light-emitting layer is formed on the surface of thelight-emitting layer, the part of the light-emitting layer correspondingto the non-light-emitting region can be allowed to emit light in thesecond light-emitting layer. Specifically, the leakage current passingthrough the surface defects from the quantum dots in thenon-light-emitting region of the light-emitting layer increases, wherebyelectric charge is concentrated in the second light-emitting layer toallow the second light-emitting layer to emit light, and light ofdifferent wavelengths can be emitted from one light-emitting element.Moreover, since the second light-emitting layer serves also as anelectron transport layer, the element structure can be simplified.

Also, when the light-emitting layer is formed of a laminated body of twoor more kinds of quantum dot layers having different average particlesizes and the light-emitting layers are laminated so that at least apart of the non-light-emitting region and at least a part of thelight-emitting region may overlap with each other, light of differentcolors can be emitted from the quantum dots of the same material by thequantum size effect from each of the light-emitting regions, so that alight-emitting element having good light-emission characteristics can beobtained at a high efficiency and with a low cost.

Also, according to the display device of the present invention, numerouslight-emitting elements described above are arranged in an array form,so that a display device such as a full-color display can be obtainedwith a low cost.

In this manner, according to the present invention, the light-emissionand non-light-emission of the light-emitting layer can be controlledsimply depending on whether a surfactant is present or not on thesurface of the quantum dots contained in the light-emitting layer, sothat a small display device having high performance can be obtained at ahigh efficiency and with a low cost.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating oneembodiment (first embodiment) of a light-emitting element according tothe present invention.

FIG. 2 is a view schematically illustrating a state in which asurfactant is coordinated on the surface of a quantum dot.

FIGS. 3( a) and 3(b) are a production step view (1/2) schematicallyillustrating one embodiment (first embodiment) of a method of producinga light-emitting element according to the present invention.

FIGS. 4( c) and 4(d) are a production step view (2/2) schematicallyillustrating one embodiment (first embodiment) of a method of producinga light-emitting element according to the present invention.

FIG. 5 is a cross-sectional view schematically illustrating a secondembodiment of a light-emitting element according to the presentinvention.

FIG. 6 is a view illustrating a light-emission spectrum obtained from afirst element part of Example 1.

FIG. 7 is a view illustrating a light-emission spectrum obtained from asecond element part of Example 1.

FIG. 8 is a production step view illustrating a patterning method ofPatent Document 1.

FIG. 9 is a production step view illustrating a patterning method ofPatent Document 2.

DETAILED DESCRIPTION OF THE INVENTION

Next, embodiments of the present invention will be described in detailwith reference to the drawings.

FIG. 1 is a cross-sectional view schematically illustrating oneembodiment (first embodiment) of a light-emitting element according tothe present invention.

Specifically, in this light-emitting element, a positive electrode 2 isformed on the surface of a transparent substrate 1, and further a holetransport layer 3 is formed on the surface of the positive electrode 2.

Also, a light-emitting layer 4 made of quantum dots is formed on thesurface of the hole transport layer 3. Then, the light-emitting layer 4has a light-emitting region 4 a that emits light of a firstpredetermined wavelength (for example, 622 nm) and a non-light-emittingregion 4 b that does not emit light.

On the surface of the light-emitting layer 4, a second light-emittinglayer 5 that emits light of a second predetermined wavelength (forexample, 525 nm) that is different from the light emitted from thelight-emitting region 4 a is formed, and further a negative electrode 6(first negative electrode 6 a, second negative electrode 6 b) is formedon the surface of the second light-emitting layer 5. Specifically, thefirst negative electrode 6 a is formed above the light-emitting region 4a, and the second negative electrode 6 b is formed above thenon-light-emitting region 4 b, whereby the light-emitting element issectioned into a first element part 8 a and a second element part 8 b,as shown in FIG. 1. Here, in the present embodiment, the secondlight-emitting layer 5 serves also as an electron transport layer.

FIG. 2 is a cross-sectional view schematically illustrating a quantumdot contained in the light-emitting layer 4.

Specifically, this quantum dot 9 has a core-shell structure made of acore part 10 and a shell part 11 that protects the core part 10, and asurfactant 12 is coordinated on the surface of the shell part 11. Then,by this surfactant 12, agglomeration of the quantum dots 9 with eachother is avoided.

In the quantum dots 9 contained in the light-emitting region 4 a, thesurfactant 12 is coordinated to be present on the surface. In thequantum dots 9 contained in the non-light-emitting region 4 b, thesurfactant 12 is removed to be absent on the surface.

Specifically, in the light-emitting region 4 a, the surfactant 12 iscoordinated to be present on the quantum dots 9, that is, on the surfaceof the shell part 11, so that the surface defects of the quantum dots 9are inactivated. Moreover, the surfactant 12 used in the quantum dots 9generally has a low carrier transport property, so that the electronsand the holes are attracted to be confined effectively in the inside ofthe quantum dots 9 and recombined in the quantum dots 9 by voltageapplication between the positive electrode 2 and the negative electrode6, whereby exciton light emission is made at a high efficiency.

On the other hand, in the non-light-emitting region 4 b, the surfactant12 is not present on the quantum dots 9, that is, on the surface of theshell part 11, so that the surface defects of the quantum dots 9 are notinactivated. Therefore, the carriers are trapped by the surface defects,and the excitons lose energy to be deactivated with no radiation tobring about a non-light-emission state. Moreover, in this case, theleakage current from the quantum dots 9 that has passed through thesurface defects increases, and electric charge is concentrated in thesecond light-emitting layer 5, whereby the second light-emitting layer 5located above the non-light-emitting region 4 b undergoes exciton lightemission. This surfactant 12 can be easily removed by a thermaltreatment around the boiling point, so that the light-emitting layer 4having the light-emitting region 4 a and the non-light-emitting region 4b can be formed simply by performing the thermal treatment on a specificregion of the quantum dot layer.

Here, the core material for forming the core part 10 of the quantum dots9 is not particularly limited as long as it is a semiconductor materialexhibiting a photoelectric conversion function, and ZnSe, ZnTe, InP,InSe, CdSe, CdS, PbSe, and the like can be used. Also, as the shellmaterial for forming the shell part 11, ZnS, GaN, AlP, and the like canbe used, for example.

Also, the surfactant 12 is not particularly limited as long as it can beeasily removed by a thermal treatment such as laser radiation, andlong-chain amines such as hexadecylamine (hereafter referred to as“HDA”) and octylamine, trioctylphosphine oxide, pyridine, and the likecan be used.

Here, the transparent substrate 1 is not particularly limited, andinorganic materials such as glass and transparent resins such aspolycarbonate and polyacrylate can be used, for example.

As the positive electrode 2, it is preferable to use a transparent andelectrically conductive material having a large work function so as tofacilitate injection of holes, and indium tin oxide (hereafter referredto as “ITO”) and zinc oxide materials such as zinc oxide-indium oxidecan be used, for example.

As the negative electrode 6, it is preferable to use an electricallyconductive material having a small work function so as to facilitateinjection of electrons, and Al and Ca can be used, for example.

Also, as a hole-transporting material that forms the hole transportlayer 3, inorganic oxides such as nickel oxide (NiO) and molybdenumoxide (MoO₃), arylamine derivatives such asbis(N-(1-naphthyl-N-phenyl)-benzidine, triphenylamine derivatives suchas copoly[3,3′-hydroxy-tetraphenylbenzidine/diethylene glycol]carbonate,N,N′-bis-(3-methylphenyl)-N,N′-bis-(phenyl)-benzidine (TPD), and4,4,4-tris(3-methylphenylphenylamino)triphenylamine, carbazolederivatives such as polyvinylcarbazole and4,4-N,N′-dicarbazole-biphenyl, distyrylarylene derivatives such as1,4-bis(2,2-diphenylvinyl)benzene, fluorene derivatives such aspoly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl))diphenylamine)],and spiro compounds such aspoly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(9,9′-spiro-bifluorene-2,7-diyl)]can be used, for example.

Also, as an electron-transporting material that forms the electrontransport layer 5, aluminum complexes such astris(8-hydroxyquinoline)aluminum (hereafter referred to as “Alq3”) andbis(2-methyl-8-quinolylato)-4-phenylphenolato)aluminum, oxadiazoles suchas (2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole),phenanthrolines such as 2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolineand 4,7-diphenyl-1,10-phenanthroline can be used, for example.

In the light-emitting element constituted in this manner, when voltageis applied between the negative electrode 6 (first negative electrode 6a and second negative electrode 6 b) and the positive electrode 2,electrons are injected into the negative electrode 6, and holes areinjected into the positive electrode 2. Then, the electrons injectedinto the negative electrode 6 pass through the electron transport layer5 to enter the quantum dots 9 of the light-emitting layer 4, and theholes injected into the positive electrode 2 pass through the holetransport layer 3 to enter the quantum dots 9 of the light-emittinglayer 4.

Then, in the first element part 8 a, the surfactant 12 is coordinated tobe present on the surface of the quantum dots 9 contained in thelight-emitting layer 4, so that the light-emitting layer 4 forms thelight-emitting region 4 a as described above and generates excitonlight-emission at a high efficiency. Specifically, in the first elementpart 8 a, excitation light from the light-emitting region 4 a of thelight-emitting layer 4 penetrates through the hole transport layer 3,the positive electrode 2, and the glass substrate 1, as shown by anarrow A, and outputs a predetermined light-emission color (for example,red light).

On the other hand, in the second element part 8 b, the surfactant 12 isnot present on the surface of the quantum dots 9 contained in thelight-emitting layer 4, so that the light-emitting layer 4 forms thenon-light-emitting region 4 b as described above. Then, the leakagecurrent from the quantum dots 9 that has passed through the surfacedefects increases, and electric charge is concentrated in the secondlight-emitting layer 5, whereby the second light-emitting layer 5located above the non-light-emitting region 4 b generates excitonlight-emission. Specifically, in the second element part 8 b, excitationlight from the second light-emitting layer 5 penetrates through thenon-light-emitting region 4 b, the hole transport layer 3, the positiveelectrode 2, and the glass substrate 1, as shown by an arrow B, andoutputs a predetermined light-emission color (for example, green light).

Next, a method of producing the above light-emitting element will bedescribed in detail.

First, a quantum dot dispersion solution is fabricated. Here, for theultrafine particles constituting the quantum dots 9, various materialscan be used as described above; however, in the embodiments describedbelow, description will be given by taking, as an example, a case whereCdSe is used in the core part 10 and ZnS is used in the shell part 11.

For example, cadmium acetate, oleic acid, and octadecene are mixed in avessel and stirred and dissolved in a nitrogen atmosphere, whereby acadmium precursor solution is prepared. Further, selenium and octadeceneare mixed in a nitrogen atmosphere, whereby a selenium precursorsolution is prepared.

Subsequently, the cadmium precursor solution is heated to apredetermined temperature (for example, 300° C.), and the seleniumprecursor solution is injected into this heated solution. Then, theprecursors having a high activity react with each other by hightemperature, whereby cadmium and selenium are combined to form a nucleusand thereafter react with the surrounding unreacted components togenerate crystal growth. By this, CdSe quantum dots are fabricated.Here, the particle size of the CdSe quantum dots can be controlled byadjusting the reaction time.

Next, a zinc oxide solution in which zinc oxide is dissolved in oleicacid and a sulfur solution in which sulfur is dissolved in oleic acidare prepared.

Subsequently, the zinc oxide solution and the sulfur solution arealternately dropwise added in a trace amount into the CdSe quantum dotsolution adjusted to a predetermined temperature (for example, 150° C.),and the resultant is heated, cooled, and washed to remove the excessiveorganic components in the solution. Then, the resultant is thereafterdispersed into a dispersion solvent, for example, into chloroform,thereby to fabricate a CdSe/ZnS dispersion solution.

Then, the surfactant 12 such as HDA is added to the above CdSe/ZnSdispersion solution, and the surface of the quantum dots 9 made ofCdSe/ZnS is covered with the surfactant 12, thereby to fabricate aquantum dot dispersion solution.

Then, with use of this quantum dot dispersion solution, a light-emittingelement is fabricated by a method as shown in FIGS. 3( a)-3(b) and4(c)-4(d).

First, as shown in FIG. 3( a), a positive electrode 2 of ITO film or thelike having a film thickness of 100 nm to 150 nm is formed on atransparent substrate 1 by the sputtering method or the like.Subsequently, by using the vacuum vapor deposition method or the like, ahole transport layer 3 having a film thickness of 10 to 50 nm is formedon the positive electrode 2.

Then, by using the spin coating method or the like, the above-describedquantum dot dispersion solution is applied onto the hole transport layer3 and dried to form a quantum dot layer 13 having a film thickness of 2to 50 nm as shown in FIG. 3( b).

Next, a specific region 13 a of the quantum dot layer 13 is subjected toa thermal treatment by laser radiation or the like as shown by an arrowC, and thus a surfactant 12 on the specific region 13 a is removed.Here, the thermal treatment temperature is preferably a temperaturearound the boiling point of the surfactant 12 that is put to use. Forexample, when HDA (boiling point: 330° C.) is used as the surfactant 12,the thermal treatment temperature is set to be 300 to 350° C. Whenoctylamine (boiling point: 176° C.) is used as the surfactant 12, thethermal treatment temperature is set to be 160 to 180° C.

By suitably selecting the surfactant in this manner, the width ofmaterial selection for the hole transport layer 3 can be enlarged. Forexample, when HDA is selected as the surfactant 12, a hole-transportingmaterial that can withstand the thermal treatment temperature of 300 to350° C., for example, MoO₃, needs to be used. However, when octylaminethat can be removed at a thermal treatment temperature of about 160 to180° C. is used, an organic compound that is inferior in thermalresistance can be used as the hole-transporting material.

In this manner, a light-emitting layer 4 having a light-emitting region4 a and a non-light-emitting region 4 b can be fabricated as shown inFIG. 4( c).

Subsequently, as shown in FIG. 4( d), by using an electron-transportingmaterial such as Alq3, a second light-emitting layer 5 having a filmthickness of 50 nm to 70 nm serving also as an electron transport layeris formed on the surface of the light-emitting layer 4 by the vacuumvapor deposition method.

Thereafter, with use of Al, Ca, or the like, a negative electrode 6(first and second negative electrodes 6 a, 6 b) having a film thicknessof 100 nm to 300 nm is formed by the vacuum vapor deposition method,whereby the light-emitting element is fabricated.

In this manner, in the present first embodiment, the light-emittingregion 4 a defining light emission and non-light-emission can bepatterned simply depending on whether the surfactant 12 is present ornot on the surface of the quantum dots 9 contained in the light-emittinglayer 4. Specifically, the light-emitting region 4 a can be patternedwithout using a cumbersome method such as the photolithography method.Therefore, a light-emitting element having good light-emissioncharacteristics can be obtained at a high efficiency and with a low costwithout the presence of a residue of the photoresist in thelight-emitting layer and without the need for consideration of chemicalresistance at the time of material selection. Also, the film thicknessof the light-emitting layer 4 can be controlled easily in the filmforming process, so that the film thickness can be formed uniformly orapproximately uniformly, and unevenness is not generated in thelight-emission characteristics. Moreover, the element will not have highresistance as in Patent Document 2, so that the element can be driven byapplication of low voltage.

Also, because the second light-emitting layer 5 is formed on the surfaceof the light-emitting layer 4, the part of the light-emitting layer 4corresponding to the non-light-emitting region 4 b can be allowed toemit light in the second light-emitting layer 5. Specifically, theleakage current passing through the surface defects from the quantumdots in the non-light-emitting region 4 b of the light-emitting layer 4increases, whereby electric charge is concentrated in the secondlight-emitting layer 5 to allow the second light-emitting layer 5 toemit light, and light of different wavelengths can be emitted from onelight-emitting element. Moreover, since the second light-emitting layer5 serves also as an electron transport layer, the element structure canbe simplified.

Here, in the present first embodiment, the negative electrode 6 isformed on the second light-emitting layer 5; however, an electroninjection layer made of LiF or the like may be interposed between thesecond light-emitting layer 5 and the negative electrode 6 so as tofacilitate injection of electrons in accordance with the needs.

FIG. 5 is a cross-sectional view schematically illustrating a secondembodiment of a light-emitting element according to the presentinvention. In this second embodiment, the light-emitting layer is madeto have a three-layer structure and has first to third element parts 14a to 14 c.

Specifically, also in the present second embodiment, the positiveelectrode 2 is formed on the transparent substrate 1, and further thehole transport layer 3 is formed on the surface of the positiveelectrode 2 in the same manner as in the first embodiment.

Then, a light-emitting layer (first to third light-emitting layers 15 to17) including a three-layer structure is formed on the surface of thehole transport layer 3. The first light-emitting layer 15 has, forexample, a light-emitting region 15 a that emits blue light having awavelength of 490 nm and a non-light-emitting region 15 b that does notemit light. The second light-emitting layer 16 has, for example, alight-emitting region 16 a that emits green light having a wavelength of540 nm and a non-light-emitting region 16 b that does not emit light.Also, the third light-emitting layer 17 is made only of a light-emittingregion that emits red light having a wavelength of 620 nm, for example.

Then, each of the light-emitting layers 15 to 17 is formed so that atleast a part of the non-light-emitting region and at least a part of thelight-emitting region may overlap with each other. Specifically, thefirst to third light-emitting layers 15 to 17 are formed so that a partof the non-light-emitting region 15 b of the first light-emitting layer15 and a part of the light-emitting region 16 a of the secondlight-emitting layer 16 may overlap with each other, and thenon-light-emitting region 16 b of the second light-emitting layer 16 anda part of the light-emitting region of the third light-emitting layer 17may overlap with each other.

Then, an electron transport layer 18 is formed on the surface of thethird light-emitting layer 17, and further a negative electrode 19(first to third negative electrodes 19 a to 19 c) is formed on thesurface of the electron transport layer 18.

In the present second embodiment, when voltage is applied between thenegative electrode 19 (first to third negative electrodes 19 a to 19 c)and the positive electrode 2, electrons are injected into the negativeelectrode 19, and holes are injected into the positive electrode 2 inthe same manner as in the first embodiment. Then, the electrons injectedinto the negative electrode 19 pass through the electron transport layer18 to enter the quantum dots 9 of the first to third light-emittinglayers 15 to 17, and the holes injected into the positive electrode 2pass through the hole transport layer 3 to enter the quantum dots 9 ofthe first to third light-emitting layers 15 to 17.

Then, in the first element part 14 a, the surfactant 12 is coordinatedon the surface of the quantum dots 9 contained in the firstlight-emitting layer 15, and the first light-emitting layer 15 forms thelight-emitting region 15 a. Specifically, in the light-emitting region15 a, the surfactant 12 is coordinated on the surface of the quantumdots 9, so that the electrons and the holes are confined efficiently inthe inside of the quantum dots 9 and recombined in the quantum dots 9 togenerate exciton light-emission at a high efficiency. As a result, inthe first element part 14 a, excitation light from the light-emittingregion 15 a of the first light-emitting layer 15 penetrates through thehole transport layer 3, the positive electrode 2, and the transparentsubstrate 1, as shown by an arrow D, and outputs, for example, alight-emission color corresponding to the wavelength of 490 nm, that is,a blue light.

Also, in the second element part 14 b, the surfactant 12 is notcoordinated on the surface of the quantum dots 9 contained in the firstlight-emitting layer 15, and the excitons are deactivated by thermalradiation to form the non-light-emitting region 15 b. On the other hand,the surfactant 12 is coordinated on the surface of the quantum dots 9contained in the second light-emitting layer 16 located above the firstlight-emitting layer 15, and the second light-emitting layer 16 formsthe light-emitting region 16 a. Specifically, in the light-emittingregion 16 a, the surfactant 12 is coordinated on the surface of thequantum dots 9, so that the electrons and the holes are confinedefficiently in the inside of the quantum dots 9 and recombined in thequantum dots 9 to generate exciton light-emission at a high efficiency.As a result, in the second element part 14 b, excitation light from thelight-emitting region 16 a of the second light-emitting layer 16penetrates through the non-light-emitting region 15 a of the firstlight-emitting layer 15, the hole transport layer 3, the positiveelectrode 2, and the transparent substrate 1, as shown by an arrow E,and outputs, for example, a light-emission color corresponding to thewavelength of 540 nm, that is, a green light.

Also, in the third element part 14 c, the surfactant 12 is notcoordinated on the surface of the quantum dots 9 contained in any of thefirst and second light-emitting layers 15, 16, and the excitons aredeactivated by thermal radiation to form the non-light-emitting regions15 b, 16 b. On the other hand, the third light-emitting layer 17 is notsubjected to a thermal treatment, and the whole region forms thelight-emitting region to generate exciton light-emission. Therefore, inthe third element part 14 c, excitation light from the thirdlight-emitting layer 17 penetrates through the respectivenon-light-emitting regions 15 a, 16 a of the first and secondlight-emitting layers 15, 16, the hole transport layer 3, the positiveelectrode 2, and the transparent substrate 1, as shown by an arrow F,and outputs, for example, a light-emission color corresponding to thewavelength of 620 nm, that is, a red light.

Next, a method of producing the above light-emitting element will bedescribed.

First, by a method and a procedure similar to those of the firstembodiment, three kinds of quantum dot dispersion solutions havingdifferent particle sizes are fabricated.

Specifically, with regard to ultrafine particles having a particle sizeof 10 nm or less, the state of electrons in the material changes andlight having a shorter wavelength is emitted by the quantum size effectaccording as the size of the particles decreases. Therefore, by usingultrafine particles of various particle sizes, various light-emissioncolors can be obtained from a light-emitting layer using the samematerial.

Therefore, in the present second embodiment, three kinds of quantum dotdispersion solutions having different average particle sizes arefabricated. For example, in the case of using CdSe/ZnS quantum dotshaving a core-shell structure, a first CdSe/ZnS dispersion solutionhaving a core particle size (average particle size) of 3.2 nm andemitting blue light having a light-emission wavelength of 490 nm, asecond CdSe/ZnS dispersion solution having a core particle size (averageparticle size) of 3.7 nm and emitting green light having alight-emission wavelength of 540 nm, and a third CdSe/ZnS dispersionsolution having a core particle size (average particle size) of 5.6 nmand emitting red light having a light-emission wavelength of 620 nm arefabricated respectively by adjusting the reaction time, and a surfactantsuch as HDA is added to these, thereby to fabricate the first to thirdquantum dot dispersion solutions.

Then, by using the spin coating method or the like, the first quantumdot dispersion solution is applied onto the surface of the holetransport layer 3 and dried to form a first quantum dot layer.Thereafter, a specific region of the first quantum dot layer issubjected to a thermal treatment, and then a first light-emitting layer15 having light-emitting region 15 a and a non-light-emitting region 15b is fabricated.

Subsequently, by using the spin coating method or the like again, thesecond quantum dot dispersion solution is applied onto the surface ofthe first light-emitting layer 15 and dried to form a second quantum dotlayer. Then, a specific region is set so as to overlap with a part ofthe non-light-emitting region 15 b of the first light-emitting layer 15,and the specific region is subjected to a thermal treatment, thereby tofabricate a second light-emitting layer 16 having a light-emittingregion 16 a and a non-light-emitting region 16 b.

Subsequently, by using the spin coating method or the like again, thethird quantum dot dispersion solution is applied onto the surface of thesecond light-emitting layer 16 and dried to form a third light-emittinglayer.

Thereafter, by a method and a procedure substantially similar to thoseof the first embodiment, an electron transport layer 18 of Alq3 or thelike having a film thickness of 50 nm to 70 nm is formed on the surfaceof the light-emitting layer 4 by the vacuum vapor deposition method.

Then, with use of Ca, Al, or the like, a negative electrode 19 (first tothird negative electrodes 19 a to 19 c) having a film thickness of 100nm to 300 nm is formed by the vacuum vapor deposition method, wherebythe light-emitting element is fabricated.

In this manner, also in the present second embodiment, a light-emittingregion defining light-emission and non-light-emission can be patternedsimply depending on whether a surfactant is present or not on thesurface of the quantum dots 9 contained in the first to thirdlight-emitting layers 15 to 17, and effects similar to those of thefirst embodiment can be produced.

Moreover, in the present second embodiment, light of different colorscan be emitted from the quantum dots of the same material by the quantumsize effect, so that a light-emitting element having good light-emissioncharacteristics can be obtained at a high efficiency and with a lowcost.

Then, in the present second embodiment, blue light, for example, can beoutput from the first light-emitting layer 15; green light, for example,can be output from the second light-emitting layer 16; and red light,for example, can be output from the third light-emitting layer 17.Therefore, by arranging a large number of such light-emitting elementsin an array form, a display device having a small size and being capableof full-color display with a low cost can be realized.

Here, the present invention is not limited to the above-describedembodiments. For example, the method of removing the surfactant is notlimited, and by using electron beam irradiation, thermal transcriptionmethod, or the like besides the laser irradiation and by performing alocal thermal treatment on the quantum dot layer selectively on onesheet of a transparent substrate, patterning of the light-emissionpattern on the same transparent substrate can be carried out.

Also, in the case of performing laser irradiation, the kind of laser isnot limited, and a laser device having an arbitrary oscillationwavelength such as a diode laser having an oscillation wavelength of 808nm, an excimer laser having an oscillation wavelength of 308 nm, or asolid laser having an oscillation wavelength of 532 nm can be used.

Also, in each of the above-described embodiments, light of two or morecolors is emitted; however, it is a preferable mode that onelight-emitting layer is provided and patterned so as to emit light of asingle color, whereby a character pattern can be displayed in a singlecolor.

Also, in the above-described embodiments, the hole transport layer 3 isprovided on the positive electrode 2; however, the hole transport layer3 can be omitted when the desired performance is not affected.

Also, in the above-described first embodiment, the electron transportlayer needs to have a light-emitting function, so that the electrontransport layer is formed of a light-emitting material. However, whenthe light-emitting function is not required as in the second embodiment,the electron transport layer can be formed of an electron-transportingmaterial that does not have a light-emission property.

Also, in the above-described embodiments, the quantum dot 9 has acore-shell structure made of the core part 10 and one layer of the shellpart 10. However, the present invention can be applied also in the samemanner to a core-shell-shell structure in which the shell part 10 has atwo-layer structure or to a quantum dot that does not have a shell part.

Next, an example of the present invention will be concretely described.

Example

A light-emitting element having the above-described constitution shownin FIG. 1 was fabricated, and a light-emission spectrum was measured.

Specifically, an ITO film was formed on a glass substrate by thesputtering method, thereby to form a positive electrode, and then theelectrode was washed.

Subsequently, MoO₃ having a film thickness of 10 nm was formed on thepositive electrode by the vacuum vapor deposition method, thereby toform a hole transport layer.

Next, Evidot600 (quantum dot structure: core-shell structure ofCdSe/ZnS, core particle size: 4.7 nm, surfactant: HDA (boiling point:330° C.), dispersion solvent: toluene) manufactured by EvidentTechnologies, Inc. was prepared as a quantum dot dispersion solution.

Subsequently, the quantum dot dispersion solution was applied onto thesurface of the hole transport layer and dried on a hot plate toevaporate the dispersion solution. Here, for the application treatment,the spin coating method was used, where the rotation number of the spincoating was set to be 3000 rpm and the treatment time was set to be 60seconds. Also, for the drying treatment, the drying temperature was setto be 100° C., and the drying time was set to be 15 minutes. Here, thefilm thickness of the formed quantum dot layer was 5 to 20 nm.

Subsequently, after the drying treatment, laser irradiation was carriedout on a specific region to perform a thermal treatment, whereby thesurfactant on the specific region was removed to form a light-emittinglayer having a light-emitting region and a non-light-emitting region.Here, for the laser irradiation, a diode laser having an oscillationwavelength of 808 nm was used, and the laser irradiation was carried outfor one minute by setting the irradiation energy to be 80 mJ/cm² so thatthe thermal treatment temperature would be about 300° C.

Next, an Alq3 film having a film thickness of 50 nm was formed by usingthe vacuum vapor deposition method, whereby a second light-emittinglayer serving also as an electron transport layer was fabricated.

Subsequently, LiF having a film thickness of 0.5 nm was formed by usingthe vacuum vapor deposition method, whereby an electron injection layerwas formed. Thereafter, Al having a film thickness of 100 nm was formedby using the vacuum vapor deposition method, whereby a negativeelectrode was fabricated. Finally, then, the element was sealed with useof a glass cap and a UV-curing resin. Then, by this process, a sample ofthe present invention having a first element part that emits light inthe light-emitting region of the light-emitting layer and a secondelement part that emits light in the second light-emitting layer wasobtained.

Next, with respect to the sample of the present invention, voltage wasapplied between the positive electrode and the negative electrode byusing a source meter, and a light-emission spectrum was measured by amulti-channel detector.

FIG. 6 shows a light-emission spectrum in the first element part, wherethe lateral axis represents a wavelength (nm), and the longitudinal axisrepresents light-emission intensity (a.u.).

As will be clear from this FIG. 6, red light having a peak wavelength of622 nm and a half-value whole-width ΔH of 43 nm was obtained from thefirst element part.

FIG. 7 shows a light-emission spectrum in the second element part, wherethe lateral axis represents a wavelength (nm), and the longitudinal axisrepresents light-emission intensity (a.u.).

As will be clear from this FIG. 7, green light having a peak wavelengthof 525 nm and a half-value whole-width ΔH of 90 nm was obtained from thesecond element part.

From the above, it has been confirmed that, with the samples of thepresent invention, desired light-emission colors that have a sharplight-emission spectrum and good light-emission characteristics and aredifferent between the first element part and the second element part isobtained.

A light-emitting element is realized in which patterning of alight-emitting region defining light-emission and non-light-emission canbe carried out easily without the need for cumbersome steps, and goodlight-emission characteristics can be obtained at a high efficiency andwith a low cost.

DESCRIPTION OF REFERENCE SYMBOLS

-   -   2 positive electrode (electrode)    -   3 hole transport layer    -   4 light-emitting layer    -   4 a light-emitting region    -   4 b non-light-emitting region    -   5 second light-emitting layer serving also as electron transport        layer    -   6 negative electrode    -   9 quantum dot    -   10 core part    -   11 shell part    -   12 surfactant    -   13 quantum dot layer    -   13 a specific region    -   15 first light-emitting layer (light-emitting layer)    -   15 a light-emitting region    -   15 b non-light-emitting region    -   16 second light-emitting layer (light-emitting layer)    -   16 a light-emitting region    -   16 b non-light-emitting region    -   17 third light-emitting layer (light-emitting layer)    -   18 electron transport layer    -   19 negative electrode

1. A light-emitting element comprising: a pair of electrodes; and atleast one light-emitting layer containing quantum dots interposedbetween the pair of electrodes, wherein the at least one light-emittinglayer has a light-emitting region in which a surfactant is present on asurface of the quantum dots and a non-light-emitting region in which thesurfactant is absent on the surface of the quantum dots.
 2. Thelight-emitting element according to claim 1, wherein thenon-light-emitting region is a thermal treatment non-light-emittingregion.
 3. The light-emitting element according to claim 1, furthercomprising a second light-emitting layer adjacent a surface of the atleast one light-emitting layer.
 4. The light-emitting element accordingto claim 3, wherein the second light-emitting layer is an electrontransport layer.
 5. The light-emitting element according to claim 3,wherein the light-emitting region and the second light-emitting layereach emit light having a respectively different wavelength.
 6. Thelight-emitting element according to claim 1, wherein the at least onelight-emitting layer is a laminated body of two or more light-emittinglayers, each of the two or more light-emitting layers having quantumdots of different average particle sizes.
 7. The light-emitting elementaccording to claim 6, wherein the two or more light-emitting layers areconfigured such that at least a part of the non-light-emitting regionand at least a part of the light-emitting region of each light-emittinglayer overlap with each other.
 8. The light-emitting element accordingto claim 6, wherein each light-emitting region of the two or morelight-emitting layers emits light having a respectively differentwavelength.
 9. The light-emitting element according to claim 6, furthercomprising an electron transport layer adjacent a surface of one of thetwo or more light-emitting layers.
 10. The light-emitting elementaccording to claim 1, further comprising a hole transport layerinterposed between one electrode of the pair of electrodes and the atleast one light-emitting layer.
 11. The light-emitting element accordingto claim 1, wherein the quantum dots have a core-shell structure.
 12. Amethod of producing a light-emitting element including at least one ormore light-emitting layers containing quantum dots interposed between apair of electrodes, the method comprising: fabricating at least one ormore kinds of quantum dots having a surfactant coordinated on thesurface; forming at least one or more quantum dot layers by using eachof the one or more kinds of fabricated quantum dots; and performing athermal treatment on a specific region of at least one quantum dot layeramong the formed quantum dot layers to remove a surfactant on thespecific region to fabricate a light-emitting layer having alight-emitting region and a non-light-emitting region.
 13. The method ofproducing a light-emitting element according to claim 12, furthercomprising forming a second light-emitting layer on a surface of thelight-emitting layer.
 14. The method of producing a light-emittingelement according to claim 13, wherein the second light-emitting layeris an electron transport layer.
 15. The method of producing alight-emitting element according to claim 12, wherein two or more kindsof quantum dots having different average particle sizes are fabricated,and quantum dot layer formation and surfactant removal are carried outeach of the two or more kinds of quantum dots to form two or morelight-emitting layers having different light-emission wavelengths. 16.The method of producing a light-emitting element according to claim 15,wherein the two or more light-emitting layers having differentlight-emission wavelengths are arranged so that at least a part of thenon-light-emitting region and at least a part of the light-emittingregion of each of the two or more light-emitting layers overlap witheach other.
 17. The method of producing a light-emitting elementaccording to claim 15, further comprising forming an electron transportlayer on a surface of an uppermost light-emitting layer among the two ormore light-emitting layers.
 18. A display device comprising a pluralityof light-emitting elements according to claim 1 arranged in an arrayform.